The present invention relates to a hermetic motor compressor unit, particularly to such a unit which is intended for use in small capacity applications, such as small refrigerators.
One of the primary concerns in designing refrigeration compressors for use in small capacity applications is that of minimizing the overall size of the unit without sacrificing efficiency or the capacity which is required. A further design consideration is that of minimizing the number of parts required and the assembly time. This is particularly important in small compressors because the manufacturing volume of such compressors is normally quite high and even small savings in material and labor reaches considerable proportions when high production levels are attained.
One of the assembly operations performed in manufacturing such a compressor is that of assembling the connecting rod to the crankshaft and piston. Because the connecting rod articulates about the piston wrist pin only in directions transverse to the axis of the crankshaft, it is impossible, when using most conventional techniques, to insert the connecting rod over the end of the crankshaft when the connecting rod is attached to the piston. One technique for assembling the connecting rod to the crankshaft is the use of a split sleeve-type connecting rod wherein the sleeve halves are assembled around the crankshaft and secured together by means of bolts. The problem with this technique is that additional parts are required and there is a substantial amount of labor in assembling the connecting rod around the crankshaft. Furthermore, the split sleeve is a difficult part to manufacture due to the necessity for accurate machining of the mating surfaces thereof.
A further solution to the problem would be to initially install the piston and connecting rod assembly into the crankcase and then insert the crankshaft through the open loop bearing end of the connecting rod. This solution is not feasible in the case of the compressor in question, however, wherein the crankshaft is disposed vertically and must have a relatively large bearing surface in contact with the supporting surface of the crankcase. This would require a correspondingly large opening in the connecting rod, which is not practical in very small compressors wherein the connecting rod is generally small. Although the connecting rod could be lengthened to accomodate the larger opening, this would increase the overall size of the compressor in the direction of the connecting rod. As mentioned earlier, minimizing the overall size of the unit is one of the design criteria of compressors of this type.
U.S. Pat. No. 3,903,752 discloses yet another solution to the problem of assembling the piston, connecting rod and crankshaft. The wrist pin and connecting rod form a unitary assembly, which is inserted into the cylinder through a slot in the sidewall thereof at the same time that the integral, open loop bearing end of the connecting rod is slipped over the end of the crankshaft. There is a corresponding slot in the piston which enables the connecting rod-wrist pin assembly to be inserted. The primary difficulty to this technique is that the wrist pin portion of the connecting rod-wrist pin assembly is not permitted to bear fully on the openings in the piston. Because a slot in the piston is necessary to permit insertion of the assembly, the wrist pin assembly bears only on the top and sides of the opening in the piston, rather than around the entire periphery of the wrist pin as in conventional designs. This presents a series problem in low temperature compressors wherein the compression ratio is much higher and, consequently, the forces between the wrist pin and piston are high. It will be appreciated that the loss of part of the bearing surface will result in higher forces per unit area on the remaining bearing surfaces. Another difficulty is the complicated structure of the connecting rod and wrist pin assembly, which makes machining more difficult. Moreover, maintaining squareness of the connecting rod relative to the crankshaft and piston is much more difficult to achieve than in the case where the connecting rod is joined to the piston by a separate, cylindrical wrist pin.
In prior art compressors of this type, the crankcase typically has been secured to the stator laminations by means of four bolts or screws positioned at the four corners of the stator. Although this provides a very stable support, it necessitates a crankcase which extends laterally over the full area of the top surface of the stator. This increases the amount of material which is required to produce the crankcase, and necessitates a generally larger crankcase.
In U.S. Pat. No. 4,115,035, a compressor utilizing a two point support is disclosed. In this case, the crankshaft extends through a central sleeve portion and downwardly extending legs at the opposite end thereof are secured to the stator by means of screws extending through the stator. It has been found that this provides a very weak support resulting in a loss of stability between the crankcase and stator. Since the rotor is secured to the crankshaft, which in turn is supported within the crankcase, any loss of stability will result in loss of integrity of the air gap. In order to maintain optimum efficiency, it is extremely important that the air gap be maintained within very precise limits around the entire periphery of the rotor.
In hermetic compressors, the motor-crankcase assembly is generally resiliently supported within the outer housing by means of spring supports. This not only isolates vibration and noise generated by the compressor, but provides some degree of isolation between the motor-crankcase assembly and shocks imparted to the housing during shipping and use.
One prior art mounting arrangement comprises a plurality of mounting spuds pressed over the heads of the screws or bolts extending through the stator laminations and resiliently retained within a plurality of respective coil springs secured to the lower surface of the outer housing. The springs are mounted to the housing by means of metal mounting spuds welded or brazed to the housing and extending axially within the coil springs. In addition to serving as the connectors to the coil springs, the spuds serve as shipping stops to limit the vertical movement of the motor-crankcase assembly within the housing.
Generally, the sockets in the upper spuds that are pressed over the heads of the connecting bolts or screws are concentric with the central axis of the spud. Because the connecting bolts or screws are necessarily disposed inwardly of the sides of the stator laminations to provide the required degree of structural integrity between the bolts and laminations, the support base for the assembly, as defined by the four support spuds, is also disposed inwardly of the sides of the laminations to the same extent. If the geometrical centers of the spuds could be relocated outwardly, then a more stable support base for the motor-crankcase assembly could be provided.
The mounting spuds and their associated coil springs present a problem in that they often intefere with the end turns of the field windings, which extend out of the slots of the stator and form a mass on the lower surface thereof. This necessitates that the end turn configuration for the field winding be carefully controlled so that the end turns do not come into contact with the springs, which may result in wearing through of the insulation and shorting of the winding.
Generally, compressors of this type are designed such that there will be no contact between the motor-crankcase assembly resiliently supported within the housing and the inner wall of the housing during normal use. During shipping of the unit, however, it is often subjected to severe shocks thereby causing the motor-crankcase assembly to strike the inner wall of the housing and cause damage to the compressor or rupturing of the hermetically sealed housing. Undue movement of the motor-crankcase assembly is also necessary to prevent overstressing of the mounting springs and discharge gas shock loop.